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On which point(s) in a jet engine does the reaction force act?
Turbojet and turbofan engines are part of a family of engines referred to as reaction engines to emphasize the fact the aircraft is pushed forward mostly by Newton's 3rd law of motion when gases are accelerated.

Diagram of a turbofan jet engine, source
On which point(s) does the reaction force act?

I'm interested in why jet engines are special in relation to the 3rd law of motion. Might have to ask a question!

@Gusdor you can still ask a new question; perhaps with a link to this one to provide context

The thrust produced by the engine is due to the net resultant of various forces acting on various surfaces of the engine. The thrust produced is a function of mass flow rate and the change in velocity ($T = \dot{m} (V_{e}-V_{\inf})$). So, both of these have to be taken into account, rather than acceleration alone. The location of maximum thrust generated varies with the type of engine.

For high bypass jet engines like those used by the modern commercial airliners, most of the thrust is produced by the bypass fan. Though the acceleration is not the greatest here, mass x acceleration is- so this is where the thrust is maximum. The maximum reaction forces are applied there
In case of pure turbojets, the (almost all of the) thrust is produced by the core. For low bypass turbofans, its somewhere in the middle, with the lion's share of thrust produced by the core.

Note that most of the reactive force is applied on the diffuser and the compressor due to the high pressure and the forward facing area (due to cross sectional variation) in this region. This high pressure also acts on the combustion chamber, increasing the reaction force.
In the turbine and nozzle, the variation in the cross section creates a surface area facing the rearward direction, where the gas pressure acts, resulting in an force applied in direction opposite to the compressor. The net result of all these forces gives the thrust.

Image from Aircraft Performance and Design by John Anderson; taken from quora.com

@ymb1: The explanation for the variable nozzle geometry is simply to balance the flue gas temperature in order to keep a constant pressure level. Speed of sound increases by the root of the gas temperature, while density decreases by the inverse of the gas temperature. Summed up, the mass flow density is roughly proportional to the rooted inverse of the gas temperature. The afterburner shall not affect upstream engine components (air mass flow + pressure need to be constant), thus a larger cross section is required. Switching on the reheat with no surface adjustment causes a pressure increase.

@aeroalias: Nice answer. In the case of the turbojet / low-bypass-ratio turbofan you could add the case of (super-)sonic exhaust stream in which an additional contribution of thrust stems from the pressure gradient between nozzle area and ambience.

This is correct for a turbojet, but totally wrong for a large turbofan, where most of the thrust comes from fan's interaction with the air that does not go through the engine core. By Newton's third law, this produces a forwards force on the fan, which is transmitted along the fan shaft to a thrust bearing, then through the bearing support structure (the rectangular pink piece in the OP's picture) and from there directly into the pylon and the aircraft wing.

@mins The answer to that depends on what sort of analysis you are doing.  In general, you can model the force coming from a point, and the results will be meaningful, especially if your stability margin is high.  However, if you're working on a statically unstable aircraft design, you may find that the analysis calls for more accurate modeling.

@CortAmmon: I agree with your comment, still as the total thrust can be represented by an horizontal vector, where is the vector origin for a turbojet and a turbofan? My current understanding is on the engine axis, near the fan for a turbofan, and between the compressor rear diffuser and the combustor intake for a turbojet (Carlo Felicione suggests a position closer to the LPT). Is this correct?

@mins since the only stress applied is a force with no torque, it's application point can lie anywhere on the centreline, it won't matter. In screw theory, it is called a sliding wrench I believe. Though good luck googling these keywords :-\

There is no torque when you express the wrench anywhere on the centreline (well, maybe some axial torque, by that can slide too). There are torques when you move the wrench off the centreline and onto the engine mount points, though, of course.

Most of the thrust is generated in the combustion chamber, followed by the compressor stages. The exhaust is only contributing a small fraction of the total forward gas load.

Source: the jet engine, Rolls Royce (ISBN: 9781119065999)
The picture show the gas load contribution of various parts of a pure jet engine. In turbofan engines, a relatively larger part is contributed by the fan.

A definition of "gas load" would be a good addition to this answer.

@pericynthion The "gas load" is the total axial force applied by the gas to all the engine parts (casings, rotor and stator blades, combustion chamber, etc). In the compressor and turbine sections, most of that force acts on the blades, not directly on the engine casing.

The reaction forces are applied, strangely enough, to the fan disk as it produces the lion's share of the thrust by accelerating the largest quantity of air flowing through it; it is the actual object making contact with the airflow and pushing it out the nacelle outlet.
The low and high pressure compressor sections have additional reactive forces imparted upon them as the accelerate an air mass through them.
Reactive force is lost as air flowing through the gas core imputs a reactive force opposing forward thrust on the turbine rotors and hot section stators - the energy is extracted in the form of mechanical work to drive the fan/LPC/HPC.
On turbojets, low bypass turbofans, and to a small extent the gas core of a high bypass turbofan engine, the majority of the reaction forces are being applied to the fwd section of the jetpipe near at the last LPT stage or stator prior to entering the jetpipe due to the imbalance of pressure between this section of the engine and the exhaust nozzle outlet at atmospheric pressure.

For the very special case of engines operating in the supersonic region, a large part of the thrust is applied via the intake.
"The Concorde Air Intake Control System" via http://www.pprune.org/tech-log/426900-concorde-engine-intake-thrust.html :

A huge 75% OF THE TOTAL THRUST is produced by the intake subsonic diffuser section, this being due to the huge rise in static pressure that is occurring in this section. The 'negative thrust' from the forward ramp section this time is 12%, produced by the supersonic compression forces acting on the divergent section of the intake, resulting in an intake thrust component of 63%. So it can be seen that the vast majority of the Mach 2 thrust forces are transmitted to the airframe not via the engine mountings, but via the mountings of the intake, and to a lesser extent the TRA nozzle.

The wording of that quote is seriously misleading. The force is indeed applied to the diffuser, but it is not generated by it. Because the reason why the thrust exists is the pressure increase in the combustion chamber and the fact that the rear "wall" holding that pressure is the accelerating exhaust stream, so the forward force gets applied to the aircraft while the aft force does not.

Let's take concept of energy for help. All jet engine thrust originate in combustion chamber (energy of burning fuel). Smaller or bigger part of this thrust ( depending on engine construction) is transferred from turbines blades ( high and low pressure) forward via axis to compressor / fan.
Let's take example of low pressure turbine/ fan system. Expanding gas in combustion chamber pushes on turbine blade and via axis rotates fan blades. So any forward force created by fan blades is transferred via axis and turbines blades back to its origin i.e. combustion chamber ( backward facing surfaces). So in theory if all rotating parts would be free to move forward during steadily running engine they wouldn't move.
So analogically to center of gravity of an object the central point of thrust within the jet engine lies in geometrical center of backward facing surface of combustion chamber.


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How often can I fly my own small plane?
I was wondering about how often can one fly a privately owned plane? Is it somehow like owning a car where you just light up the ignition and just drive away? Obviously, there are a lot of regulations and rules... but say I have my plane parked in a hangar in a nearby airport. Can I just drive up to that airport and take my plane for a spin anytime I want (after coordinating with the control tower)?

I think you may read about class G airspace. Otherwise, at least for ultra light in Europe, you can see the hangar as a shared private parkiing attached to a club (with its own rules)

"Whenever your wallet allows"... which when you own a small aircraft is much less often than you may think.

Welcome to aviation.SE! See the information in this question about part 91 flights too; private flights are usually operated under part 91 regulations.

@boodo17, Are you just asking hypothetically, or do you actually own a plane?

If you're going to "take [your] plane for a spin", just make sure it's certified for spins. ;-)

Depending on what country you live in, yes, but it will cost you.
It's probably the cheapest to fly an airplane you own in the United States.  Here, the answer is yes, with very little restriction, you can just drive out to the airport and fly an aircraft within the National Airspace System.  You just must abide by the legal and medical requirements for you, and the legal and operational limitations of the aircraft.  Most airports throughout the country are uncontrolled, so you may have limited, if any, interactions with ATC.  The ability to fly pretty much anywhere you like with very few restrictions is what makes GA so appealing in the United States.

Why is it the cheapest to fly in the US? Nothing prevents the owner of a private plane living, for instance, somewhere in Europe to do exactly the same

@J.Hougaard Fuel/Landing fees/ATC fees are cheaper/non-existent in the US for most GA. Fuel is generally cheaper in the US, most airports don't charge landing fees for GA aircraft, and ATC is a service that isn't charged for here.

@RonBeyer Apart from fuel potentially being cheaper, those other points you mention also apply in Europe.

This varies heavily by country, and I know some countries have almost no general or private aviation. But in the UK, USA, Australia, most of Europe etc yes owning a plane is almost like owning a car in terms of freedom to use it.
Certainly in the UK, where I'm from, you can take off from a private strip in uncontrolled airspace with no radio and no flight plan. Flying is heavily regulated, but a lot of non-pilots are astonished at how easy it is to fly and how little oversight there is once you have a valid licence and aircraft.
Even using a club aircraft is often less onerous than renting a car.

say I have my plane parked in a hangar in a nearby airport, can I just drive up to that airport and take my plane for a spin anytime I want (after coordinating with the control tower i guess)?

Absolutely - weather and local rules depending, this is exactly how many private pilots operate. At my airport, cooridinating with the tower is as complex as 10 second phone call to tell them you're going flying. Or, you can even do it over the radio though that's discouraged.

I'm curious why you say that talking to tower over the radio is discouraged.

@MichaelHall Sorry I could have been clearer - we have a "booking out" process, where you let the tower know the information about your flight. Persons on board, destination etc. You can "book out" over the radio, but they prefer you to phone with those details to save some airtime. It's pretty common in the UK at towered fields - there's normally some kind of process or sheet to sign. Some have a dedicated office linked to the tower who will pass over your flight details

@Dan That sounds like what in the US would be called “filing a VFR flight plan”. It’s not required and the only time I file one is if I am flying into a Presidential Temporary Flight Restriction.

In the US, you can pretty much legally fly your private plane whenever you want, as long as the ceiling and visibility are compatible with flight under Visual Flight Rules.  If you have an instrument rating, then you have even more freedom, but you have to follow certain procedures in order to exercise that freedom.

Carlo Felicione is exactly right. His answer is spot on. In the US, you can fly at any time, day or night, as often as you would like. Every day if you want. Just as long as both you and your aircraft meet the minimum legal requirements to fly.
An easy memory aid to remember the legal requirements is:
Pilot

Must be in possession of proper license, medical certificate, and ID.
Must also be current on the operation of the aircraft.
Illness - free from any affecting flight.
Medication - restricted
Stress - Be mentally fit to fly.
Alcohol - Effectively, none allowed
Fatigue - free from any affecting flight.
Emotion (hard to legally quantify)

Aircraft

Documents - E. A.R.R.O.W. C.C.C.
Inspections - A.V.1.A.T.E.S., Preflight
Maintenance - ADs, 337, MELs, KOELs, TSDS Life Limited Parts
Equipment - Primarily 14 CFR Part 91.205

enVironment

NOTAMs (for TFRs, NavAid outages, construction, etc)
Weather at departure, enroute, destination, and alternates
Known ATC delays
Runway lengths of intended airports of use
Alternatives to your plan (alternate airports or transport)
Fuel requirements of flight
Takeoff and Landing performance calculations

Extenuating Circumstances

Flight conditions must be within your Personal Minimums.

There are so many airports without control towers that you would not need to coordinate with anyone. An airport with an operating control tower would control their respective airspace. But, working with them is relatively easy. Typically, you only need to notify them of your intent to fly once you have started your aircraft engine and are ready to taxi. They will then give you clearance instructions. If you intend to fly IFR, you will need to file a flight plan at least 30 minutes ahead of time. If you intend to fly VFR, no flight plan is necessary. Although, one is highly recommended.
Just like driving a car, you are free to fly an aircraft whenever, wherever (taking into consideration airspace, restrictions, and safety of flight) and however often you like. Unlike a car, there is a lot more planning and prep work that is required to go into each outing. And, there are a lot more and a lot stricter rules of the sky than there are rules of the road. You can’t legally just jump in your airplane and go for a spin. At least here in the US, that is.

"You can’t legally just jump in your airplane and go for a spin. At least here in the US, that is."  Yes you can.  I fly out of a non-towered airport, under Class B/C airspace, I can go to the airport, prep the plane, fire up & go. Don't have to talk to anyone, but announcing intentions and listening for other is always a good idea (like stopping for STOP signs on the road before pulling out).  Keeping the plane airworthy and documents up to date is similar to a car, keep those current and just go. If you know the plane and the area, a briefing is usually a good idea, can be online, or a call.

@CrossRoads - I doubt most pilots would check the weather each time they jumped in their car for a fifteen minute drive to their local Wal-mart. Neither would they do a full pre-drive inspection. And, I certainly don’t calculate my expected fuel burn regardless of how far I am going to drive. Although, keeping track of your documents and inspections should be a no-brainer and done over time. Some things like NWKRAFT are mandatory for each and every flight, required in part by Part 91.103. Other things may not be mandatory. But, thorough flight planning is never a bad idea.

As far as flying out of a non-towered airfield and staying VFR in Class G airspace (or even E) outside of any Mode C veil, I already covered that in my answer. Now, if you are just beating up the traffic pattern at your local Class G for hours on end, you can get away with a lot less planning and prep work. But, I doubt that is what you are doing.

https://www.law.cornell.edu/cfr/text/14/91.103  "become familiar with all available information concerning that flight."  NWKRAFT covers that, in way too much detail for every flight.  If I'm just going for take off and landing practice at my home airport in my personnel airplane, then a look at skyvector.com for any surprise TFRs (President/VIP flying to an airport nearby?) and a glance at the weather can be enough, along with normal pre-flight actions, like checking for external damage, dipping the tanks and checking the oil.  Going elsewhere requires a little more, or if using another plane

I'm with CrossRoads on this one.  Yes, you can legally jump in your airplane and go for a spin any time you want, which is the basic question.  Sure, there are things you can and should do first to be legal and responsible, but there is no government permission required to exercise your freedom.  (God I love this country!)

@Dean F.: But I don't do a lot of thar every time I fly, especially if it's locally - or if I do, it's just in my head.  Check the weather?  I do that by looking outside (though I do live where it's easily predictable most of the year).  I'd check if I was going on a longer trip, of course, but I'd do that if driving, too.  Calculating fuel burn is a matter of putting the measuring stick in the tanks and seeing how much I've got - just as I'd look at the car's fuel gauge.  And I check the car's oil, tire pressure, and so on regularly.

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Using the APU for heating on the ground with engines shut down
I would like to refer one technical question to you that I couldn’t able to explain myself. It is from the FCOM Supplementary Procedures Chapter of a Boeing 737 Classic. This question concerns the use of APU as a heat air source on the ground with engines shut down. The supplementary procedure explains that under extremely cold weather conditions both packs may be used for more rapid heating.
It directs to open the Isolation Valve Switch which is reasonable. However it also says that during right pack operation only, under cold conditions, if the left PACK TRIP OFF light illuminates, position the recirculation fan OFF until the cabin temperature stabilizes.
Here, I don’t understand do they mean that left pack is INOP? I am confused because this procedure explains that both packs may be used!? And the other thing, why should I position the left recirculation fan to of if the LEFT TRIP OFF light illuminates under these conditions. As far as I know the recirculation fan reduces the air conditioning pack load, or am I missing something.

Something is off there - you won't get a Pack Trip on the left pack if it's already turned off. Reread the section and confirm that you aren't mixing up left/right or pack trip vs bleed trip someplace. Also, a pack trip happens when the pack is working hard to cool, not heat, the air. When heating a cold aircraft, a duct overtemp is the most likely light you'll get.

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Are Uber Elevate and other air taxis subject to domestic ownership and control rules?
When Virgin Airlines USA was being established, the company was forced to restructure due to a determination that effective ownership and control was held by Richard Branson's Virgin Group, a non-US entity. These rules are widely criticized in many countries, but existing carriers like the reduced competition they provide and lobby to keep them in place.
Uber plans an air taxi service and has recently sold a significant share to Softbank, a Japanese company (Fortune). Uber as a ride hailing service has worked around many conventional taxi regulations, but as it gets into air taxis it will be up against the US Department of Transportation which is not so easy to push around. If Elevate is considered an air taxi it will have to respect air taxi rules.
Are US air taxi companies subject to domestic ownership and control rules just as domestic airlines are?

The most important thing to remember is that currently there is only one FAA regulation about person-carrying autonomous drones: it is simply not permitted.
Both 14CFR Part 121 (airlines) and Part 135 (charter/air-taxi) have ownership and citizenship requirements. (It is of course not known what Part person-carrying drones would be under, but we can assume similar requirements.)
However why should this affect Uber? They do not own the cars that are currently in the fleet; in exactly the same way they could have the aircraft be owned not by Uber, but by some sub-contractor.

What are the part 135 ownership and citizenship rules? I was thinking about this in the Uber context, but rather than generating speculation about how Uber would be classified I asked only about US air taxi operators.

But the sub-contractor, under the Uber model for cars, would have be a private aircraft owner hiring himself out under the Uber banner.  Which would require the private owner to get an OC and commercially register his aircraft yadda yadda yadda.  That won't work. And if Uber is going to subcontract with existing air taxi operators, that would be like them contracting out to auto taxi companies.  Doesn't make a lot of sense.

What do you mean "that won't work"? The question here is about Uber and its foreign investors (Softbank), not about any sub-contractors. Yes of course the sub-contractors would have to conform to whatever citizenship/ownership regulation ultimately gets passed. (Incidentally, Uber does also use auto taxi companies.)


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Is the F-16 capable of taking off during a nerve gas attack?
Imagine a scenario where air bases are attacked with (or indirectly affected by) a nerve agent during a major war. There is also a desperate need to scramble combat aircraft. Can modern combat aircraft, such as the F-16 still take off during such circumstances? What about other typical modern combat aircraft (F-15, F/A-18, F-22, F-35)?
While higher up in the air, I imagine the nerve agent should pose no threat to the pilot. However, it appears that even just suspecting there is nerve agents in the air on the runway should be life threatening to the pilot. Wouldn't the life support system ingest the nerve agent and present it to the pilot through the same system?
I haven't previously read about and couldn't find anything about chemical warfare filters equipped on the F-16's life support system. Does it have any?
However, I glanced at this info card from Cobham on F-16 Support Equipment:
On-Board Oxygen Generating System.
It seems that the F-16 equipped with OBOGS has an on-board emergency oxygen supply. From various forum posts it also seems like the older block C F-16 has emergency oxygen supply. It seems reasonable that all combat aircraft have some sort of emergency oxygen supply, for obvious reasons. Correct me if I'm wrong.
Perhaps naively, it seems like enabling the emergency oxygen supply while taxiing and takeoff for the F-16 would allow it to still get in the air? Is there enough time to do this? Is the emergency oxygen supply stored pre-pressurized or does it require to become pressurized by the running engine? In that case, I suppose this is a futile effort. This is all of course, assuming a rather unlikely scenario where the hangar for the aircraft is assumed safe enough to even start up the aircraft in the first place.
What if the hangar is also unsafe? Can anything be done for the pilot and crew, beyond this? I suppose the crew could wear hazmat suits, but again, what about the pilot? For some aircraft, such as the SR-71, I believe pilots wear pressure suits before getting into the aircraft. However, then I assume the aircraft is then required to support this type of equipment, which I doubt the F-16 does.
Does any nearby nerve agent attack effectively ground all aircraft? It seems like something there should be a solution for.

I don't know about the F-16 specifically, but NATO air bases in West Germany in the 80's used to regularly hold exercises under full NBC (Nuclear-Biological-Chemical) conditions, with everyone in gas masks and protective suits. I'd be very surprised if they'd go to the hassle of protecting the ground crew, fire trucks, base security and runway repair crews, but forget the pilots.


text:
Which aerodiesels comply with MOSAIC?
MOSAIC will, presumably, expand the list of type certified aircraft operable by a pilot with a sport license.
Current Light Sport regulations result in a countable number of type-certified aircraft legally operable by a light sport pilot - almost exclusively tail dragger soft-bodies: Aeronca's Chief, Superchief, and Champ; Ercoupe 415; Interstate Cadet; Luscombe 8; Piper Cub and Cub Special; Porterfield Collegiate; Quicksilver GT500 (Technically a kit, but they'll sell it to you ready to fly); and Taylorcraft's Cub, B, D and L-2 Grasshopper.
The MOSAIC expanded list is basically anything with a clean stall 54 knots or less - and will include several purpose-built aircraft designed to break into the anticipated market.
A comprehensive list of MOSAIC-compliant type certified airplanes would be too long to ask for, but I'm interested in learning more about single prop aerodiesels (either stock or with STC that doesn't cost as much as the plane itself) that would be flyable under MOSAIC - if any yet exist.
When I attempt to look up single prop aerodiesels, I'm unable to easily find their clean stall speed (Vs1).  For example, the Tecnam P2010 gives a stall speed, but it's flaps down, power off, which is not part of the MOSAIC rules.
I'm am looking for a list of MOSAIC-compliant, a consistently easy way to find a given candidate aircraft's clean stall speed, or specific candidates that meet the criteria even if the list is not exhaustive (in that order of preference).
The criteria are:

Single-prop
Four seat max
Clean stall 54 knots or less


text:
How does maneuvering affect radar detection of stealth aircraft?
Stealth aircraft like the F-22 and F-35 have very low radar cross sections. These planes have angles of their fuselage set-up in a way to deflect radar signals. As far as I understand another stealth property is that the aircraft itself is relatively flat. When it comes to maneuvering and for example rolling the plane do these features still apply, as the "flat" effect is pretty much gone? So how does maneuvering affect radar detection?

RCS depends on the aspect presented to the radar source, so an aircraft can offer a low radar return from one angle and a large one from another. How much the RCS changes by depends on how much work went into optimizing the airframe from all sides.
Since reduced RCS tends to be at odds with aerodynamic performance, not all sides of an aircraft will be optimized, which will affect mission planning: with proper intel, an aircraft can plan its ingress and egress vectors in such a way to present its "best profile" to any hostile radar installations.
As an example, compare the F-22 with the Su-57. Both airframes have had some low observability built into them, but to different degrees. In particular, the rear aspect of the Su-57 is pretty conventional, suggesting that for whichever reason (funding, time limits, serial production costs, contract requirements, etc) it was deemed acceptable to have a larger RCS from behind.


text:
Why did the CAA not use Monarch aircraft to repatriate stranded Monarch passengers?
This week Monarch airlines went bust, leaving hundreds of thousands of passengers stranded abroad.
The CAA has been secretly planning for some weeks a repatriation 'airlift' in case the airline folded. Apparently they are chartering 30 or so planes from airlines including Qatar Airways for this.
Since Monarch is now bust, all the Monarch aircraft are sitting idle on the tarmac. Why not use those planes to repatriate people? Surely, it would be a lot cheaper and easier than chartering a whole bunch of other aircraft. They could even use the Monarch crews, who are all now unemployed, after all.

I suspect this has very little to do with aviation as such: it's a legal/financial issue. With Monarch in administration, who owns those aircraft? If the CAA wants to lease them, who do they talk to? Who would insure the aircraft and the operations? The staff are unemployed (I assume), so would the CAA have to employ them as employees or contractors? What are the payroll, legal and liability issues of doing that? Now, all those things could be solved given enough time, but with thousands of people stranded you need a quick and reliable solution, which is chartering in this case.

@Pondlife: If Monarch owned the planes, you'd talk to the administrator (KPMG). Legally, they must extract as much value from the remaining assets as possible. If leased, the owners aren't getting any money anymore, and would presumably be happy with a short-term lease. Insurance is not a real problem for a government.

Pondlife pretty much hit the nail on the head.
When a business goes bankrupt or bust, depending on jurisdiction, the assets of the company may not belong to the company anymore, they may belong to creditors. Monarch may not have even owned them in the first place, since some airlines use leases from other airlines or banks to get aircraft.
Even if Monarch did own the aircraft, and they went belly-up meaning the aircraft were basically nobody's, now you have to worry about insurance and liability. The aircraft need to be insured to fly, and there is an assumption of liability if something (like a crash) were to happen. There are logistics like maintenance, repairs, etc. Somebody will have to go through all the aircraft and make sure that they are legal to fly (review logs, maintenance records, etc).
Then you have the crews: they are not employed, and the CAA isn't going to employ a bunch of pilots for a couple of weeks just to run ferry flights. The pilots may demand benefits, etc. It would be difficult or impossible to contract them as private parties (since most insurance companies would not let you do that).
So the easiest option is to let somebody else take care of all that by using charter flights. They have the infrastructure (pilots, planes, fuel contracts, insurance, etc) to get these people out. The CAA just has to call them up and say "we need X number of people picked up from ABC on some-date", and the charter company takes care of it. Of course it is a little more expensive than doing it yourself, but you aren't trying to basically build a short-term airline and then shut it down again.

Would it be possible that they got them relatively cheap, chartering at the last minute (given they're likely going to be sat around idle if not)? Especially with the Qatar aircraft chartered, which have mostly been doing nothing since the Saudi airspace ban.

Unlikely, doing the math... The article linked in the question says it will cost about $61m GBP or $80.3m USD to get the approximately 110,000 passengers repatriated. If you work it out, that averages $730 USD per seat for a one-way flight. Not much of a deal if you ask me.

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text:
Is there a "pounds per hour" reading that could have signaled the fuel leak to the pilots of Air Transat 236?
Watching a documentary and reading up on the Air Transat 236 incident, I would have thought that even with the tech available in 2001, it would have been simpler to spot a fuel leak, even with the strange warnings going on. Don't aircraft have a "pounds per hour" real-time display that indicates rate of fuel expenditure?
If so, would such a device have been taken to have malfunctioned or showing incorrect information, or would the difference it showed at the time of the leak be unnoticeable? It is said that the pilots were checking fuel balances half hourly or so before the first alarm. Would a "pounds per hour" display be a good up-to-the-second indicator, that even if the last fuel check was ok, there was now a steady problem? Would it remove be necessary to make any other calculations or do any other guesswork? Was this figure/display not available on this flight?

Fuel flow is measured by flow transducers in line with the fuel pipes. And they do a very good job at measuring flow, but unfortunately a fuel leak bypasses them. There is no direct measurement of the leak flow anywhere.
The pilots of Air Transat 236 had two indications of the fuel leak:

Increased fuel flow through the oil/fuel heat exchanger caused a drop in oil temperature and a corresponding increase in oil pressure.
Drained fuel on the leaky side caused a fuel imbalance.

It would only be possible to positively identify increased flow as a leak if the leak occurs in between two flow transducers, both mounted in the same fuel line. If the leak occurs after a single transducer, the increased flow gives an indication. Otherwise, the leak flow would have to be observed from available volume, minus legitimately used engine flow. There are fuel tank probes distributed in airliner fuel tanks that give an indication of fuel volume per tank. Picture below shows the six fuel probes in the 747 centre tank (source).

The difficulty here is that leak flow must be derived from volume measurement: unit of flow is $m^3$/sec, the derivative of volume. The wing tanks are long and flat and the probes measure vertical distance, with an accuracy that suffices for Qty indication but lacks the precision of the fuel flow meters which directly measure impulse of the mass flowing through them. Plus they measure mass directly, a more useful entity than volume. Volume per mass changes with fuel temperature, tank levels change with aircraft pitch and roll, the level needs to be derived over time: all of this can only give an indication over the course of hours, when a large amount of fuel has leaked out already, and with a degree of uncertainty that would leave room for doubt.
You're right though, in wondering why there seemed to be no automated way for monitoring discrepancies in fuel quantity. @Waked mentions the estimation messages of the B737NG, which would also only seem to occur after much fuel has been lost aready. The A330 now has an ECAM caution message: FUEL FU/FOB DISCREPANCY, put in place after the Air Transat incident. Lessons learned from this incident would now be incorporated in the fuel management systems and aircrew procedures. The brilliant glide flight of the pilots demonstrates once again why we need pilots in front of the aeroplane.

Fuel qty discrepancies are indeed monitored in some aircraft. The following applies to B737 NG: FMC will present the message "USING RESERVE FUEL" if predicted landing fuel is below the minimum FMC reserve fuel (typically final + alternate fuel). The prediction is based on FQIS measured fuel - FMC predicted fuel burn. When predicted landing fuel @ dest. is less than zero, "INSUFFICIENT FUEL" is shown. An "IMBAL" caution is presented if sufficient discrepancy exists between left and right fuel tanks. The relevant QRH checklist instructs pilots to confirm no fuel leak exists before crossfeeding.

@Waked, well, the A330 checklist at the time of that incident did say not to use it if suspecting fuel leak. They even tried to look whether they are leaving a trail behind them, but they couldn't see it in the dark of the night. And what the checklist did not say, and they did not make that connection, was that if the imbalance does not improve in a couple of minutes, it should be considered a sign that you do have a fuel leak and turn it off and start planning a diversion.

@JanHudec The purpose of my comment was to state that some form of "automated" monitoring does exist, not to in any way evaluate the performance of the crew.

@Waked, yes. The crew can't be really blamed for not realizing the imbalance, if it does not fix quickly, does imply a fuel leak. The author of the check-list had a lot more time to think about it then they and didn't realize either (and therefore any automated system couldn't help either, because automated system can only do what the author of the requirements thought about).

You don't need two transducers to see a leak, but the real problem is whether the leak occurs before or after the transducer which is measuring the flow to the engines...

@Lnafziger yes you can see increased flow if the leak is after the transducer, but is it due to a leak, or to an abnormal condition of engine fuel consumption? Good point, have incorporated.

@Koyovis: Either way, you're losing fuel too quickly and need to divert.

@JanHudec: ...why else would there be a fuel imbalance, assuming competent fuelling personnel?

@Sean, the engines can be burning it at different speed for various reasons—but all I can think of would either be caused by the crew or at least the crew would know about them well before the fuel imbalance is detected.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Benefit of a deswirl vane for a turboshaft helicopter engine
I am trying to understand the benefit of deswirl vanes on a turboshaft helicopter engine. For these type of engines, the kinetic energy of the exhaust stream is essentially wasted energy. Also, raising the static pressure at the exit plane by deswirling will reduce the shaft output power. So, why bother?

Could you provide an example picture for better understanding? Deswirl vanes are usually a feature of a (centriugal) compressor, but it seems you are talking about the turbine/exhaust...

@Zeus. The question is asking about the benefit of de swirl vanes at the exit of a turbine, for a turboshaft engine. i.e. why not just have the last rotating stage as the final set of aerofoils, why add the weight of stationary exit guide vanes as the last stage of the LPT.

This is a very good question. However, there is a slight error in your question, which is causing the confusion.
Turbine power, is determined by the ratio of total pressure across the turbine (and other factors). See the formula here on NASA’s website.
$$ W_{Turbine} = \eta \cdot c_p \cdot T_{t_4} \cdot \left( 1-TPR^{\frac{\gamma-1}{\gamma}} \right) $$
Where $$ \gamma = \frac{c_p}{c_v}$$
and TPR is the total pressure ratio across the turbine:
$$TPR =\frac{P_{t_5}}{P_{t_4}}$$
In comparison, thrust is determined by the static pressure. See the “engine thrust equation” (the second formula) on this page here. The thrust is given by two terms; one due to the increase in velocity of the mass, the other is the “pressure thrust” term, which is due the the nozzle area x the difference in ambient static pressure and the nozzle exit static pressure.
In simple form;
$$F_{N} = \dot{m} \cdot (v_e - v_i) + A_j \cdot (p_{s_{exit}} - p_{s_{ambient}})$$
(Ignoring the mass of the fuel, which is very small compared to the air mass flow, $\dot{m}$).
So, increasing the exhaust nozzle static pressure at its exit increases the thrust from the exhaust gas, but will not reduce (or change) the power provided by the turbine, because that’s a function of total pressure.
That’s not to say the amount of thrust you will get is very large, and worth the weight of nozzle guide vanes at the exit. Some turboshaft engines have them, some don’t.

Please use MathJax to format your formulae. I've converted it now, please check if I made any mistakes.

@ROIMaison.  Thanks very much, they are all correct.

I was thinking along these lines, but it doesn't really answer why would exhaust 'deswirl' vanes be beneficial. My speculation is that they are more of a structural element than aerodynamic one. Exactly because of what you say (1st formula), and because for helicopters we don't want any jet thrust (we want the turbine to take all the available pressure), it is beneficial to expand the exhaust so that the exit total pressure equaled the ambient (or even overexpand a bit!) Hence efficient helicopter exhausts tend to be large, and need support.

Also, when we speak about jet thrust, it's worth noting that exhaust velocity (1st component of the $F_N$ formula) is the main and most efficient contributor to thrust. It normally makes sense to adjust the nozzle such that all the excess pressure turned into speed. In that case, one would think, deswirling could bring benefit of straightening the stream velocity. Yet, even when such vanes are present on jet engines, they also look more like structural elements than anything else.

@Zeus what "jet thrust" are you presuming for a turboshaft engine on a helicopter?  The point of that engine design is to get as much power extracted as is feasible well before exhaust to drive the rotors.  Rotor blades/rotor systems are what provides thrust for a helicopter.

@Korvin, the answer we are commenting to talked about jet thrust amongst other things, to make a point of difference. In my earlier comment I noted that helicopter turboshafts don't need jet thrust (unlike others, including turboprops), and this causes them to have larger exhausts.

@Zeus there are a lot of novices and utterly unlettered in matters aviation who come here.  I sometimes feel the need to point out the obvious for their benefit. The question is an yet another obvious case of doing no research done before asking a question, but apparently this particularly SE puts up with that.

@Zeus.  You make a valid point that a helicopter engine is designed such that as much power as possible is removed by the power turbine. But even when that is done there is likely to be energy left in the gas stream. So, this might as well be converted to thrust as best it can. As the answer illustrates, this conversation to thrust does not come at the expense of optimising the turbine power extraction. So, it’s not a matter of wanting thrust, but a matter of using it because it’s just wasted otherwise. I also agree the vanes may have a structural role. Cheers

Thanks for the discussion - Penguin. However, for reasons below, I am not convinced of your explanation.
The formula for turbine power you quote is for a turbofan engine. For a turboshaft engine, where the exit kinetic energy is wasted, the pressure ratio and the efficiency used is inlet total to exit static. In addition, in your thrust equation above, you are assuming that the normal to the exit plane of the duct is in the flight direction which may or may not be true. Frequently, these ducts are turned upwards, sideways, etc. and in such cases, increasing the static pressure will not increase thrust.
Still at a loss on how these de-swirl vanes work for turbo shaft engines.
Sam

Hi @sam,  With your first comment, the formula isn’t specific to a turbofan, but rather is generic to any turbine. I am not sure why you think it is, maybe you could clarify? I agree completely with your second point about the direction of exhaust. It did cross my mind, so, maybe there is another reason for deswirl vanes, but what I have said isn’t technically wrong, it just might not be the complete answer to your question. Cheers.


text:
Why do modern fighter aircraft still use transparent canopies instead of cameras feeding imagery to screens?
The F-35 loves to boast about the HUD letting pilots see 'through' the aircraft, but I was wondering why they even have a transparent canopy instead of embedding the pilot deep in the fuselage like the BAe P.125 (https://www.reddit.com/r/WeirdWings/comments/b240pq/bae_p125_the_cancelled_british_equivalent_to_the/)?

So, like  a  drone except the pilot is at risk ?

First thought: what if something fails (e.g. due to combat damages)? Second thought: why just remove the canopy? Why not remove the pilot? Then I remembered UAV exists

Ejecting a canopy that heavy would be fun!

What would be the benefit of having a screens instead of a canopy? I can think of several obvious drawbacks and risks, but no apparent benefits

The forthcoming X-59 QueSST will be without front-view canopy and with a camera. Not a fighter, but still!

This is actually a great question. With the current status of AR and VR, I think we are really close to getting this to work. This will save a ton of drag while significantly enhance the pilot's vision, e.g. multi-spectrum imaging and optical-digital zoom. Another application is for tanks where the crew already have very limited visual. If the next generation fighters are still maned, then I believe this would be a key defining feature.

There is no major problem to be solved by moving the cockpit inside.
The canopy adds minimal drag, considering how much drag the whole of a fighter's airframe produces. It also offers the highest quality of vision.
To expand on the latter, let's define a few items that go into visual quality:

Field of view
Acuity, including in motion
Noise
Dynamic range
Reaction speed

The canopy's field of view is about half the sphere, while cameras can cover most of the sphere. A win for synthetic vision.
Good human vision (healthy young pilot) offers enough acuity to distinguish high-contrast objects down to about 0.1 milliradian in size. The best commercial video camera heads offer 4.5K*2K to 8K*4K resolution. To cover 12.6 steradian with 6 cameras, restricting each camera to a ~2:1 rectangle, somewhat correctable with an anamorphic lens... It's a textbook math problem, but we have the advantage of knowing it's not solved in a mathematically optimal way in real fighters using it.
For a "cube with overlaps" solution, which is viable for a fighter, the FOV of each camera needs to be about 1.6 radian vertically. This results in a best-achievable instantaneous resolution of 0.5-0.7 milliradian for Arri sensors or 0.4 for the RED (which has other drawbacks). These are commercial cameras that are miles ahead of the F-35's DAS, but indicative of what could be used in a fighter designed today and meant to fly in the 2040s.
This can be improved, 6 cameras are not some hard limit, but you're looking at 100+ to fully match the acuity of the human eye (a more accurate term than resolution, since the eye doesn't perceive the whole FOV simultaneously). At that point the system won't be reasonably compact and easy to add anymore.
At their maximum resolution, all cameras produce significant amounts of noise.  So does the human vision, but its noise is filtered out very well by the brain and almost eliminated in the macula. For small megapixel-chasing cameras, the noise is so heavy that their effective resolution is reduced several times. Above we assume professional cameras with full-format sensors. At any rate, camera+eye give more noise than just the eye.
The dynamic range of human vision, at 20 stops, is better than the best cameras at 15-16 stops, and much better than LCD at 10-12 stops (20 for OLED). But this is acceptable in daytime, and will mostly matter at night, where a fighter can rely on IR systems instead. The night-time performance of IR cameras, even though they lose a lot in resolution, gives synthetic vision a win on this.
For reaction speed, at their open gate resolution, cameras offer a best-case framerate of just 60. I won't copy the full math, but combined with a 60 fps display, this results in a minimum lag of ~40 milliseconds. This sounds acceptable at first. The real pain comes when you consider the motion resolution resulting from this low framerate.
To explain motion resolution, see how a moving picture looks on a display. This can be better demonstrated with a few illusions. Unless the motion is very slow, your motion resolution can drop from the camera's 8Kx4K to only 100-600 lines depending on the rate. In practical terms, you're going from discerning objects at 0.4 milliradian all the way to 2-10.
This won't matter much in level flight, but the moment a pilot with synthetic vision begins to maneuver, they will suffer major degradation of their vision. Video gamers learn to compensate by turning instantly and returning to a slow-moving image that can be seen at sub-1000 fps, but a plane doesn't do things instantly. Whether the ability to see while spinning will make a difference, losing it can affect pilot behavior just by trying to avoid that motion-induced blindness.
All in all, you've reduced the pilot's visual acuity, added noise, and greatly compromised their motion resolution - for what? We're not chasing Mach 3 anymore; it won't help against Mach 8 SAM. We've learned to make canopies stealthy through shaping and RF-reflective metal coating, so there's no need to shy away from one. We've learned to project images onto a transparent visor, so we can get the benefits of night vision and extra FOV without losing natural vision.
Most importantly, in the last 10 years we've also learned to protect data with authenticated encryption and send it fast over vast distances. So if things like best possible acuity, motion resolution, reaction speed, or tolerance to avionics damage don't matter much, there's no reason for the pilot to be inside the aircraft at all.

Don't spend too long at that testufo site. Your eyes won't like it!

@FreeMan Hey, at least I don't have to spend all my flying time testing that UFO!

Unless you can have a continuous display over the entire field of vision WITH fail-safes I dont think it'll ever happen.
Anything less will either require transitioning from one display(screen) to another or from display to display. If you've ever used a car GPS you will know the lag in  shifting from the in-car display to outside view.
Mark 1 eyeball is still the best!

It seems a bit disingenuous to compare a car GPS to a fighter jet's sensor fusion suite...

@Algebraist indeed, the car probably has much more recently-designed systems.

In addition to all the other reasons:

Every camera-to-screen system has lag. Lag in a dogfight can be fatal.
There will be some distortion between the camera and the screen, no matter how well designed. Distortion can be fatal too.
Cameras and screen cannot reproduce depth perception (yes there are ways to do it but they are a LOT more complicated than cameras and screens). Lack of depth perception can be fatal in lots of situations.
It is possible to get positional cues (especially depth cues) for a visible object by moving your head, even the small amount of movement that a fighter pilot's head can make. A camera screen system won't do that.

A screen system can be useful if the alternative is not being able to see in some direction at all, but you wouldn't replace direct vision with it.

The human eye is 576 Megapixels.  When a helmet mounted "virtual reality" type of helmet display comes out some day, in conjunction with cameras and hardware that can record and process at that resolution, then you might see a canopy-less fighter.  By that time, the pilot will be software, or at minimum, located remotely anyway.


text:
How does the DA 40 NG automatically control the mixture, pitch and power by just providing one throttle?
I'm a student pilot and here at my school we have the older gen DA40s which have individual lever for mixture, pitch and power.
I was looking at this video of the DA40 NG, and got quite curious as to how everything from prop to mixture is automatically controlled, and on top of that you don't need to do any run up checks!

Tried using google but nothing really came, so I thought this would be the right platform to ask and satisfy my curiosity.

Don't worry for Google. Soon, the first result you'll get will be this question.

According to Diamonds website the NG engine is a:

Austro Engine AE 300 turbocharged common-rail injected 2.0 liter diesel engine with 168 hp and EECU single lever control system

The prop is a:

3 blade MT hydraulic constant speed propeller features advanced blade geometry for efficient performance, low vibration and noise. It is automatically controlled by the engine’s digital engine control through a conventional hydraulic governor.

From the sounds of it, it is as close to a full FADEC (Full Authority Digital Engine Control) unit as you can get
It is controlled by a computer module just like your modern fuel injected car.

The question isn't how does it work, it's more why it isn't on all airplanes. It's really the same engine technology that has been in widespread use in cars for 40 years.

@GdD when an "engine controller" goes on a car one is basicly helpless.  The aviation version is redundant, but there are still risks of this happening in mid-air.  Engine controllers (and turbos) have done wonders for engine performance and reduction of emissions, and adding in prop pitch control with fuel/air control just makes it better.

The Austro engines have dual engine controllers @RobertDiGiovanni, you test them both as part of the run-up procedure. Even if both controllers fail the engine will go into an emergency mode and still run. The caveat to that is electrical power, a DA-42 went into a field after the battery connection got loose, after that they added a second battery.

@RobertDiGiovanni: Modern cars have a limp-home mode; I already encountered that in my old Ford Focus diesel from 2000. It probably would break emission norms if tested at that point, but for airplane use it's sufficient to get you back to the ground under power.

@MSalters  If an aero engine ran at the same power levels as your Focus (for limp-home mode) the pilot would be likely to have a very bad day.

It is probably a nonissue. A couple of things that worry me about this engine are that this engine appears to have a reduction drive gearbox and only one generator. Even with the dual backup batteries, these two items seem to be points of failure. I wonder if there is a way to add a second generator.

@MikeBrockington: It lost ~50% of its power. I think larger two-engined planes should even be able to continue a take-off when that happens? But in general it gives you far more flexibility in selecting a landing site. The idea is to turn a very bad day into a rather annoying day.

@MikeBrockington  It'd be like losing an engine on a twin, except without the asymmetrical thrust problems.   Anyway you can have computer control without computer total control; for the first 10 year of computers all they did was fine-tune; they did PWM to a mix-control solenoid on the carb, which tweaked the mix to keep the mix stochiometric based on what the O2 sensor was saying; but its limits of travel were sane driveable values.  You could remove the computer and drive almost normally, maybe get 1 MPG worse (so 6 MPG instead of 7 lol).

@GdD, for various reasons, the light-aircraft industry is pretty much stuck in the 1960s.

Aside from the fact that the engine is computer controlled...
the reason you don't have mixture control is Diesels don't use that.
Gas engines need to breathe a stochiometric mix of air and fuel, i.e. proportioned correctly so there's just enough fuel for the oxygen admitted.  Too fuel-lean and it won't burn.  Power is controlled by partially blocking the air intake with throttle plates (hence, vacuum).  Getting the mix stochiometric under all conditions is a hard job, and that's what the carburetor does. The mixture control helps you fine tune what the carb cannot.
In a diesel, you gulp in a full shot of air (throttle plates: Gone), and compress to a high compression ratio and high heat. This means when fuel is injected, combustion is both certain and spontaneous.  So they don't need spark plugs, and can happily run lean (mixture control: Gone).  Spontaneous means the engine intake drawing in a fuel-air mixture is out of the question; it would ignite too soon. The engines are direct injection.
Diesel power is controlled solely by how much fuel is injected per cylinder. That is done at the fuel injection pumps. One pump per cylinder that pumps at the appropriate time in the stroke, driven by a cam.  Each pump has an adjustment that decides how much fuel is injected. And this is controlled by a "fuel rack" which sets all pumps the same, and the fuel rack is the "throttle".
I always thought diesel's simplicity would make it ideal for aircraft, but the high compression ratio requires stronger cylinders and makes diesel engines heavier, and that's an issue.
Of course nowadays, in their mad pursuit for EPA Tier 4 emissions, they use computers. One common trick is to eliminate the mechanical cam and rack with sensors and EFI to inject at  the right time.  But they don't have to do that.  It's a reliability vs emissions numbers call.

As stated above in the last answer, it’s turbodiesel powered.  Like the DA-42 and DA-62, the engine is basically power by wire, using a primary and secondary Engine Control Unit (ECU) computers, each with a dedicated battery backup, both for engine control and control over the constant speed propeller.

Other answers concentrated more on the 'mixture' and FADEC side of things. I'll show what it does with the prop speed. This is actually simpler and is explained right in the flight manual (p. 7-25).
The prop speed is pre-programmed to follow the engine lever:

So, it only uses highest RPM at takeoff power, and then reduces it at cruise settings. At very low settings it 'unloads' the prop by commanding higher RPM (which it may not reach). This may increase windmilling drag though, and this is a common complaint about automaitc prop control.
It is not necessary to have FADEC for such programming. Cirrus aircraft use mechanical linkages to achive a similar control.


text:
Does Daylight Saving Time affect the flight time?
Say an example flight FRA-LHR, was scheduled say 9am every day. Now will that flight be then moved to 10am once the change to DST happens or will it remain at 9am?
I would expect the flight times schedule to remain the same i.e. the daily morning FRA to LHR flight will be always departing at 9am (independent of summer/winter time changes). My knowledge in this area is limited so I wanted to know what the experts on this site say.
My question is more directed at the flight schedules i.e. the daily flight scheduled at 9am for example - does it undergo schedule change and is moved to 8am/10am or is this time fixed (underlying assumption being: in general a flight is scheduled for a certain time to meet a demand, so moving the local time is not necessarily feasible).

Your intuition is correct, airline schedules (nor virtually any other) are not set by the sun. I've had many appointments at 12pm local time, but never an appointment at local solar noon.

The more interesting situation is when a flight operates between a country which uses daylight savings time and another country which doesn't.  Or when the two countries switch over on different dates (i.e. USA and Mexico switch on dates that are a week or three apart).

As others have pointed out, in cases where both origin and destination shift to DST, there likely isn't a change.
When only one end of the flight shifts to DST it does have an effect on aircraft scheduling, so it will likely result in a schedule adjustment. A good example can be seen on Delta's daily flights between Detroit and Shanghai. China does not observe DST, so Shanghai is 13 hours ahead of Detroit EST and only 12 hours ahead of Detroit EDT.
In the summer with Detroit in EDT, the schedule is:
DTW - PVG  Flight 583    Dep: 4:39 pm  arr: 6:55 pm
PVG - DTW  Flight 582    Dep: 11:45 am  arr: 1:48 pm
and in the winter with Detroit in EST, the schedule is:
DTW - PVG  Flight 583    Dep: 3:30 pm  arr: 7:20 pm
PVG - DTW  Flight 582    Dep: 12:20 pm  arr: 1:16 pm

Very interesting since neither shift is 1 hour in an attempt to keep at least one end anchored. They have shifts in scheduled times at both departure and arrival. Weird...

Also, the flights take longer in the winter than the summer. I guess that's gate-to-gate time, so they allow an extra hour for de-icing and other delays in the winter?

Remember that these long overseas flights often affect other scheduled flights that are connectors so there is a ripple effect through the schedule. And wind patterns shift between winter and summer.

thank you FreeMan & Gerry for you answers - appreciate the help.
Just to share with you some of my understanding - a somewhat random time schedule change effect can be observed in EU market as well - which aligns with Gerry's arguments. Random in the sense that some flights move forward 30mins some 50mins etc.

It will affect scheduled flight times the same way it will affect your favorite TV show's air time.
i.e. it won't*
It will affect apparent flight duration if the departure and arrival airports switch to DST on different days of the year, but actual time in the air will remain the same.
* The caveat for that is if you happen to be in an area that A) does not switch to DST and B) your favorite TV show happens to be broadcast from an area that does switch to DST. (This situation happened every year in Indiana for many years until IN joined the 19th** century and switched to DST in the summer.)
** Yes I know that DST was a 20th century invention, it's just that Indiana is a bit behind the times...

@mins some states don't want DST because they're afraid the curtains will fade from the extra hour of daylight.

Round here, the general excuse was "the cows won't know when to give milk".

thanks for the answer @FreeMan and sorry i probably did not make myself clear. I am aware that actual time in the air is the same, however my question was more directed at the flight schedules i.e. the daily flight scheduled at 9am for example - does it undergo schedule change and is moved to 8am/10am or is this time fixed (underlying assumption being : in general a flight is scheduled for a certain time to meet a demand, so moving the local time is not necessarily feasible)

judging by your answer this is indeed the case

i am interested if anyone else has a different point / counter point in the answers

As am I, @davidski, which is why it's generally recommended not to accept the very first answer right away, which you didn't. Gerry has provided a counter point with an actual example. It appears that the real answer is "They don't have to, but they might." i.e. #ItDepends


text:
Why do military pilots report "gear down" during their traffic calls?
I've noticed that military pilots will often say something along the lines of:

<airport> traffic, Basher 5-2 left base, full stop, gear down.

Why the additional "gear down" call?  Shouldn't it be implied that if you're landing full stop, you'd want the gear down?  And similarly, if the gear isn't down, wouldn't an emergency call be more appropriate?
To be clear, I'm not asking why it's a good idea to check to see if the gear is down, I'm asking why military pilots broadcast the fact that they've checked.

Just musing, it makes sense to me that coming back from a combat patrol which saw action could leave a pilot in a state which is slightly more frazzled than you would expect from an average cross-country civilian haul.  And while it's always inexcusable to crash land because you forgot your landing gear, the cost for a civilian is measured in dollars of lawsuits.  Messing up a runway in combat could cost a whole lot more.

A long time ago I was idly watching a customer from a southern European air force playing with his new aircraft, which he was due to fly back home the next day. He flew in a circle, put his wheels down, rolled along the runway, lifted again, put the wheels up, repeat. Till the moment he touched down without putting the wheels down. We all looked at each other "there goes our Christmas bonus." This is why.

I don't have enough basis to submit this as an answer (I'm not a pilot, nor military) but from military novels written by ex-pilots, I seem to recall the following: putting the landing gear down disables the weapons systems. It's a fail-safe measure to make sure nothing accidentally goes off during landing. As such, you'd want to call it out as it's an important safety step.

@SouthShoreAK On some aircraft, it vents the aux tanks.

Other answers mention "it's the rules" without specifying why the rules are what they are.
Civilian airplanes are for the most part assumed to be in proper working order after a flight, unless it is known otherwise. Military flight hardware may have experienced 8-9 G loads, supersonic airspeeds, ground fire, attacks from other aircraft, been subject to an EMP, and likewise for the frail meatsack at the helm. One cannot assume that all components are working properly after a mission.
Additionally, civilian aircraft usually do not carry high explosives which could turn a belly scrape into a fireball. Recently a (non-American) military aircraft had an ordnance issue when landing (gear down) with undeployed payload.

Related; in most dual cockpit aircraft the PF will command "gear down" and the PA will confirm with "gear down, checked green". It is also part of the landing checklist - but obviously not annunciated in single cockpit scenarios.

This seems to be the most appropriate answer.

Why do military pilots report “gear down” during their traffic calls?

Because it's what they are trained to do.  (Example of a regulation here).  I spent a few years teaching  military flying.  Military aircraft have been flying for over a century.  There have been enough gear up passes1 over the years that the habit pattern of checking and calling the gear (in any aircraft with retractable landing gear, which is the vast majority of military aircraft, and all primary trainers) has become a standard.

I've noticed that military pilots will often say something along the
  lines of:  traffic, Basher 5-2 left base, full stop, gear
  down.

Not understanding how this is a problem.

Why the additional "gear down" call?

It's not "additional."

Shouldn't it be implied that if you're landing full stop, you'd want the gear down?

It's not a matter of "wanting" the gear down, it's a matter of reporting that you have lowered the gear and that you have checked and confirmed that it is down.

And similarly, if the gear isn't down, wouldn't an emergency call be more appropriate?

Not by default.

If it isn't down because of an oversight, it would be appropriate to
lower it and then either go around and try again, this time with the
gear down, or if confirmed down before landing, land.  That will
vary with the situation and the SOP of a given squadron/wing.
If it won't come down, then you don't call for a landing until you have tried to get it down, possibly making a low pass to see if
someone can provide you with a gear check (maybe your indicator is
being tempermental) and then, if the gear won't come down, declare
an emergency and attempt the procedures for your gear up landing.  Usually you'll be asking for such crash and rescue as is available to be ready, in case things go sideways.

1 A gear up pass, what is it?  An attempt to land with the gear up when they should have been down.

@DavidRicherby It absolutely is the answer.  Training.  Something maybe you don't get about military aviation.  Then read the part about retractable gear and building habit patterns.  If we do it at home, if Hurts Nothing to do it on the road.  That you don't care for it is an irrelevant opinion.

@KorvinStarmast New question: Why are civilian pilots not trained to do that?

@immibis suggest you ask that as a separate question.  (My first aircraft ever was a Varga Kachina, civilian flying .  Gear was not retractable.  I don't recall "calling the gear" being part of my instruction )  Reminder: this is a Q&A site, not an internet forum.

I think it's pretty clear that the question is really asking why they're trained to do that -- that is, what actual purpose the statement serves. Surely if someone asked "why do people have to learn how to fly before they can become fighter pilots," you'd be able to give a better answer than "because the regulations say so."

@immibis, actually, civilian multi-crew pilots are trained to do it, but since they are multi-crew, it is enough they say it to each other. But many military planes are single-crew, so the controller fills in for this. Civilian, only small aircraft are single-crew, so less is at stake.

@yshavit Please read the entire answer.  The reason isn't complicated, and neither is the answer.

@DavidRicherby Read sentences two, three, and four again.  If you don't know what a gear up pass is, I can't help you.  The only thing superfluous here is your commentary.  Give it a rest.

@KorvinStarmast - You're right, the question was unclear. I agree that checking and calling the gear is (and should be) the standard, both for civilian and military. What I don't understand is why the military chooses to broadcast the gear call over the radio.

@SteveV. I put the simplest piece of the answer, which is not complicated, in bold.

There are a lot of "safety" procedures in the military that aren't done in the civilian world.  Even in the civilian world there are differences.  Each airline even has its own safety procedures and even write their own manuals.  "Check Gear Down/Gear Down" is just one of them that the military established long ago as a precaution to avoid stupid mistakes that are very avoidable.  Call "Gear Down" and actually look at the gear lights to be sure you have 3 green. No one wants to be the one idiot that forgot the gear. As has been mentioned, in two pilot civilian cockpits this is done between the pilots. It gets very busy in the traffic pattern and stupid mistakes happen.  "Gear Down", as well as the many other SOPs, keep it all standardized and helps make sure that its always done the same way by everyone, everywhere.  In a huge organization like the USAF such things are very helpful when you have personnel constantly training, coming and going and transferring etc.
Similar calls are "call the ball/roger, ball", "feet wet/feet dry", "fence in - check fuel flows are good, fuel levels are good, IFF is stdby, oil pressure good, fuel tanks and oil system pressure off ( in case you take a hit ), PC system all good, weapons selectors set, weapons master off, chaff/flares are armed, etc..." (when crossing the hypothetical "fence" into hostile territory when on a combat mission ).  Another neat one is calling "Base Plus" when climbing above Controlled Airspace above 60,000 feet which can still be heard from time to time.

For USAF pilots of retractable gear aircraft, it is a required call. Per Air Force Instruction (AFI) 11-202 Volume 3, General Flight Rules:
7.7. Landing Gear Reporting Procedures. Retractable gear aircraft will report gear down
status to ATC or runway supervisory unit after extending the landing gear. This report shall
be made during any approach prior to crossing the runway threshold.

It is not a procedure exclusive to Military aircraft, also gliders report "gear down and locked" when entering the downwind. After up to 12hrs of flight the operational fatigue may lead to dumb mistakes, it is not uncommon at all to see gear up landing amongst those who think they are too cool for the checklist.


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Is the F-117 fleet at Tonopah Test Range being mothballed?
Are the F-117s based at Tonopah Test Range still in service? I heard from some reputable sources they are still flying around and being maintained, but someone else told me they are going away permanently. Does anyone know what's up with the F-117s right now?

The Lockheed F-117 Nighthawk article on Wikipedia has a whole section on "Later service and retirement" that details the current status with many references.

Wiki articles were interesting. There is something up with the F-117 ibut I don't know what. Just trying to find out anything anyone knows. In the end is usually about money and contracts. There are portable toilets at IceCap on the old Nevada Test Site.... the test tower was put up in 1992 over the hole. I would like to have had the contract to sevice the "AndyGumps" ... they are there for the test site tourists I guess. IceCap is the last underground test still standing on the now NNSS.

Some F-117s under storage (not in active service) were still flying sometime back because their storage type required for it. Attributed to USAF by http://intercepts.defensenews.com/2014/11/we-now-know-why-the-f-117-is-still-flying/:

Per Congressional direction within the FY07 National Defense Authorization Act the aircraft were placed in Type 1000, flyable storage for potential recall to future service.  In order to confirm the effectiveness of the flyable storage program, some F-117 aircraft are occasionally flown.

The same article gives some details about the type 1000 storage:

Aircraft in Type 1000 storage are to be maintained until recalled to active service, should the need arise. Type 1000 aircraft are termed inviolate; meaning they have a high potential to return to flying status and no parts may be removed from them. These aircraft are ‘re-preserved’ every four years.

However, this may soon change as the House Armed Services Committee’s version of the National Defense Authorization Act repeals the 2007 provision for storage:

This section would amend section 136 of the John Warner National Defense
  Authorization Act for Fiscal Year 2007 (Public Law 109-364) by striking subsection (b), which would remove the requirement that certain F-117 aircraft be maintained in a condition that would allow recall of those aircraft to future service.

So, while F-117s are not in USAF service, some were still flying (until recently) though they are probably going away for good soon.

I am going to find you a link, since there is a play to retire 4 per year for the next few years until they are all divested, per the 2017 defense authorization act (DoD budget).  There is some 2.1 billion dollar project that allegedly moves some of these aircraft to aliied air forces.  will get back to you

Being demilled link is here.  I need to ask a few old buds, I saw an article about some 2.1 billion dollar project on the F-117 that alluded to some of our allies getting some, but now I can't find it.  Will follow up.


text:
Where are the grounded 737 Max planes being kept?
With various countries grounding the 737 Max, where are they (their carriers) keeping the grounded planes?  Are they simply being kept at Gates of various airports (which I doubt since gates are expensive and/or logistical reasons)? Are there "holding areas" at some airports?
While I understand some carriers just fly a few of these, what about those carriers with larger fleets, such as ones in China or the EU?

Gates (with jetways and all that) are extremely precious.  They are only used by passenger aircraft, and even then to load or unload passengers.  My last flight had a nice tailwind and arrived early; we had to wait 20 minutes for a gate to open up.
During any other times, aircraft are sent to a ramp area, or a chunk of airport area reserved for parking and servicing airplanes.
London Heathrow is a very cramped airport with an abnormally tiny ramp area, however here is much of it.

Note the special guest near bottom, just right of center.
Notice a couple of areas walled off for engine run-up testing.  Southwest of here there's additional ramp in the space reserved for Terminal 5D.  All in all, Heathrow is very sparse on ramp area; even SFO has more.  (SFO also still has the literal ramps for seaplane operations.)
Now, busy airports have lots of taxiways that less-busy airports do not need, like taxiway A and B at Heathrow, which provide basically 2-way traffic. If an incident causes reduced activity that makes some of those taxiways redundant, they can park planes on them.  Further, if certain runways are expected to be crosswind runways for the duration of a crisis, they can stack those up too.  At extremes, Kingman Airport basically took their east-west runway out of service and is using it to store decommissioned RJ's. They used to store more aircraft still.  This could also be done temporarily with a crosswind or little-used runway.

Also, even if an airplane is grounded, a ferry move may be permitted out of a very cramped airport like Midway to an airport with more ramp space available.  Look at this sat photo of the ramp at Victorville (former George AFB).  See something familiar?

The date is Aug. 25, 2018, and these planes are about 110' long, making these most likely 737-300's (or -700s?).  Bet Southwest wishes they had them back right now! The one at far right is about 129' long, making it a -800 or MAX 8.

One of those planes is not like the others. One of those planes isn't the same!

@DavidRicherby sssshh! It's incognito.

By the way, although the photo clearly shows Southwest-liveried Boeing 737s, I don't think we can identify what specific kind of 737s they are. Since this answer was posted within a few hours of the grounding of the American 737 Max fleet, and since Google maps satellite views can be years old, I think it's misleading to suggest that the photo contains Max 8's. In particular, the shadows of the wings suggest that these planes don't have the split wingtips that are characteristic of the Max-8.

@DavidRicherby: Besides, many if not most of Southwest's 737NGs have also been retrofitted with split winglets (albeit of a slightly different design).

Indeed, according to Great Circle Mapper, there were no 737 MAXes at Victorville as of 13th March.

@DavidRicherby I believe that's a DC-10 Super 73.

Those aren't 737NGs, Southwest hasn't displaced any from the MAX yet. Those are 737 Classics which were taken out of service because of the fuel tank interting requirement (December 2017).

@user71659 I see the -300 is the same length, but those planes really look like they have winglets.

@Harper Winglets are available as a retrofit on the Classics.

@Harper I guess you mean DC-8-73 (the -10 was a trijet).

@DavidRicherby Oh sure enough.  The odd ones, DC-3, DC-5, DC-9 are all twins.  DC-4,6,8 are all 4 engine.   So should the DC-10.  The big trijet should have been the DC-9-1/2 :)

As reported on 23 March, Southwest was ferrying 737 MAX planes to Victorville, CA.

They're at whatever airport they were at (or next landed at) when they were grounded. Airports are big enough places that there's room plenty of room to park planes that aren't currently in use, and it's completely normal to have such planes parked at an airport. Just look at a few in Google Maps satellite view.
At least in Europe (and presumably elsewhere), the directive banning flights does allow one ferry flight (of up to three legs) to get the plane to a location where any corrective action can be taken. But note that this does not allow moving the plane just to get it to a more convenient storage location.
Great Circle Mapper's featured map for 14th March showed the actual location of the 737 MAXes owned by US airlines, as of the previous evening.  Those aircraft are spread across around 30 airports in the US and Caribbean.

Thanks to Antzi for pointing out the ferry flight exception.

Yeah, this coupled with the fact only 376 are out there, answers it for me. Thanks!  And I suppose airports that do have the 737 landing there are almost by default big enough to have space to park them out of the way, and wouldn't be smaller regional places without such space.

@BruceWayne It depends where you are in the world. In the US (where 737 MAXes are still flying, anyway), there are huge fleets of regional jets that are significantly smaller than a 737; in Europe, major airlines mostly don't operate a whole lot of planes that are smaller than a 737/A32x.

Some countries allow a limited number of ferry flights tho.

@Antzi If you have a source for that claim, please post it as an answer. I'm kind of surprised by it, though: the concern is that the planes will fall out of the sky and that's still a problem even with no passengers on board.

@DavidRicherby: Many, if not most, of them'll have to be ferried anyway sooner or later in order to get them to maintenance bases where they'll be modified to comply with the new 737M ADs, and, with a ferry flight, there're only two people - the captain and first officer - at risk (assuming that, if another one does fall out of the sky, it'll do so over an unpopulated area, since cities are tiny compared to the world's land area as a whole), compared to hundreds for a regular passenger flight.

@DavidRicherby I won't post it as an answer since it doesn't really answer the question as stated. Source (EASA): https://ad.easa.europa.eu/blob/EASA_AD_2019_0051_E.pdf/EAD_2019-0051-E_1 relevant part: From the effective date and time of this AD, do not operate the aeroplane, except that a single non-commercial ferry flight (up to three flight cycles) may be accomplished to return the aeroplane to a location where the expected corrective action(s) can be accomplished.  Maybe you could edit your answer ?

@Sean given that the modification will most likely be software only, it can probably be facilitated almost anywhere there's ground power available.

@Antzi Thanks -- edited. I wonder if one can fly a 737 Max from A to B today, claiming that as the first leg of an A-B-C flight for maintenance...

@Sean You're making bad assumptions. The two 737 Maxes that have crashed so far have done so shortly after take-off. Airports are, by design, close to urban areas so, assuming the two crashes weren't just a coincidence, a crash is much more likely to be in an urban area than the global average. Even the average can be pretty high: nearly 11% of England is classified as urban. And you can bet that, if there are any 737 Maxes at Heathrow, they won't be allowed to ferry on a day when the wind's from the east so take-offs are over central London.

Airports are, by design, close to urban areas - While that may be true in the UK, North American airports tend to be located far from the urban areas they supposedly serve (with a few exceptions, such as KBOS and KLGA).

@Sean Sorry but that's just wrong. JFK and EWR are 13 miles from Manhattan and surrounded on three and four sides by heavily urban land; ORD is 15 miles from downtown and surrounded on all sides; ATL 8 miles, ditto; LAX 11 miles, 3 sides; SEA 11 miles, 4 sides. Even DEN, which is famous for being in the middle of freaking nowhere is less than 20 miles from downtown. For reference, Lion Air 610 crashed 22 miles from the airport and Ethiopian 302, 39 miles. If an airline flies 737s to an airport in the US, you can pretty much bet that half a million people live within 20 miles of that airport.

FlightRadar24.com has compiled a list of where (almost) all the grounded 737 MAXs were as-of 17 March 2019.
Their page shows airports with 5 or more aircraft and there is a list that's sortable by registration, aircraft type, MSN, airline and airport.  The airports with the most are:

Guangzhou Baiyun, China (15)
Boeing Field, WA, USA (14)
Ürümqi Diwopu, China; Windsor, ON, Canada (12 each)
Dubai, UAE; Istanbul, Turkey; Beijing Capital, China; William P. Hobby, Houston, TX, USA (10 each)

good edit, David. I shoulda done that myself...

You're welcome. You'd been flagged, presumably as "link-only", but I think it's a link worth preserving.

Bloomberg has an Interactive Map showing where the planes are in the USA.
You can scroll through the days, and it also shows which flight paths they took to arrive at their "final" destination, and is color coded by carrier.
Pretty neat!
Edit: Also, they're storing some in the Boeing Employee's Parking Lot:


text:
Why would a commercial flight make banked turns five minutes before landing / low altitude?
As a passenger on a commercial flight into Essaouira From London Luton a short while ago, I noticed the aircraft made several quite steep banked turns, all to left, very close to landing. When I say close I mean about five minutes before we touched the runway, and I'd estimate 600-1500 feet off the ground at most. It was possible to make out individual bushes in detail in the scrub. What was more concerning was that the terrain became undulating and was rising into rounded hills with a height difference of around 250 feet.
Sky was clear, and I don't remember any turbulence near the airport.
Is it normal to bank that close to the ground? I thought an approach was lined up further out and higher up.

Essentially all turns in an aircraft are banked, so your question is just "Why did the aircraft turn before landing?" and the answer is "So it was pointing at the runway."

@DavidRicherby I think more realistically it was "Why did the aircraft make such turns so low and so late?"

This landing from the South includes reversing course by 180° at about 5 km from the threshold with two 90° left turns. Does that match your observations? Indeed you are close to the Atlas mountains. At 5 km, with a 5% slope, you are  250 m / 800 ft above the ground.

Related: http://aviation.stackexchange.com/questions/1598/why-are-vectors-used-for-sequencing-rather-than-speed-control and http://aviation.stackexchange.com/questions/2143/why-did-this-aircraft-re-route

I highly doubt that the airplane was 600-1500 5 minutes before landing.

I agree with @GdD. 5 minutes and 600-1500 ft AGL don't go together. The descent rate in approach is normally somewhere in range 600-800 ft/min, so either it was 5 minutes, but then more like 3000-4000 ft, or it was 600-1500 ft, but then it was only 1-2 min. The later would be unusual, but the former would be quite normal point for turning to runway heading.

Flying into Boston in the late 1970's a maneuver like that usually meant that the Allegheny Airlines pilot had just noticed Logan Airport passing beneath his wing.

Thanks from your comments, much appreciated on my first Stack Exchange post. Landing from the south does indeed match my observations, thank you @mins - I've edited my question to say all turns were to the left.  Jan Hudec: it could have been later than 5 minutes, I didn't take the exact time sadly, but quite definitely the flight was at less than 3000 ft AGL to my eyes.

As denizhanedeer said, it was a circling approach. There is only one charted IFR approach to ESU. It comes in from the northwest and would line up for a straight-in approach to runway 16.

Depending on the wind the pilot may have needed runway 34 (same runway from the opposite end) so they would circle around to line up with it.
This flight did just that. (It might even be your actual flight, Easyjet 2039 on 5/17/2016) It did a short hold at the final approach fix to lose altitude then circled to land on runway 34. It landed at 4:09 UTC. It did the first left turn at 4:06 and 1850 ft. The image here shows it during the second turn at 4:07 and 1475 feet.

The altitudes given are MSL. The terrain rises pretty quickly there. I'm guesstimating from some topo maps that at the area of the turns it rises to 175-200 feet. So 1475 ft MSL would be as low as 1275 ft above ground level.

Thanks for your detailed answer. That wasn't my particular flight but an approach to runway 34 seems to match my observations. My flight was 30th APR-16 EZY2039.

Now, I'm not an ATP, but as a pilot I know that sometimes I (and commercial airlines) will make "S" turns on final for spacing.  There are minimum times between an aircraft taking off/landing and another aircraft landing behind them due to wake turbulence.  Instead of making the aircraft go-around (which would take quite a bit of time), they may be asked to slow down as much as safely possible and make the s-turns for spacing.

making S turns with airliner is not even a subject. Especially in Luton.

@denizhanedeer He was departing from Luton, not landing there. But, yes, airliners do sometimes make s-turns for separation if the aircraft landing in front of them is being slow about clearing the runway. Here's an example of a 747 making s-turns for that reason. That said, it doesn't sound like the turns in the OP's question were s-turns to me. Sounds more like they were just flying the approach (and the OP's estimate of height was probably off.)

Given that the sky was clear and there are no charted visual approaches in the Moroccan Aeronautical Information Publication for Essaouira (GMMI) -- what I suspect happened was that ATC gave your flight a vector or vectors to a visual final approach after having them descend to the area Minimum Vectoring Altitude.  (The Minimum Vectoring Altitude for an area can be lower than the published procedure altitudes.)
Also: while "normal" approaches line up early, airspace or terrain constraints mean there are procedures that call for turns much lower and later than the vectoring your flight receieved.  Examples include the main instrument approaches to runway 19 at Reagan National (KDCA).

From some quick Google Earth measurements, there is some terrain to the south of GMMI that would likely call for some vectoring if landing to the north.

I don't know the type of aircraft but most probably you were not so close to ground. In normal stabilised approach it would be approximately 1:30 minutes from 1500 ft to ground. In old days we were making timing approaches for defined ground speeds.
So Second part : Airliners don't make step turns unless otherwise there is emergency. Bank limit for turn 33 degrees for airbus and after that you need to push even more to 67 degrees without slip control. If you make a step turn so close to ground you need to apply a lot of thrust due to load factor to achieve level flight. Also you definitely feel the turn due to increased G.
My option, you may have seen a nice circle to land operation, wind condition may not be the best option for approach runway, either pilot or ATC may ask to circle another runway which is close to ground, but again in safe altitude. 500 ft above the ground an airliner must be aligned and stabilised, unless safety issues.
Above requirements are for conducting a safe flight, depends on carrier and pilot they may have push limits more if you think that you were closer than 500 ft.
Especially in Africa, there are many non-precision approaches and you may need to make approach to one runway and circle to land another one.
I could not find official charts of ESU. Never been there but hope it helps.
Cheers,